ELECTRODEPOSITION OF BLACK OXIDE COATINGS ON ALUMINUM 6061 IN DEEP EUTECTIC SOLVENTS, FOR SOLAR THERMAL COLLECTION APPLICATIONS

摘要

The aqueous deposition of metal and metal oxide films by electrodeposition or by electrophoretic deposition process (EPD) is economically attractive, however coatings on oxygen sensitive substrates are particularly difficult. A typical case is the aluminum substrate, where an oxide layer is easily and spontaneously formed at room temperature on its surface. Recently, a novel class of solvents for electrodeposition named deep eutectic solvents (DES) based on mixtures of choline chloride and hydrogen bond donors has been developed. These solvents are able to solubilize many different metal salts or metal oxides and show air and moisture stability offering economically viable liquids when aqueous medium is not suitable. The electrodeposition of black oxide coatings on aluminum 6061 substrates was studied using different metals salts in a Deep Eutectic Solvent (DES) as electrolyte. The electrolyte was prepared using choline chloride (ChCl) and ethylene glycol (EG) in a molar ratio of 1:2. Salts of Ni, Cr, Co and Cu were combined with Zn salts in different concentrations. An electrochemical study was performed in order to determine the best conditions for electrodeposition of black coatings using a potentiostat/galvanostat. The electrochemical techniques were voltammetry and chronoamperometry. The optical properties of the coatings were characterized using a UV/Vis spectrophotometer, fitted with a 150 mm integrating sphere for the determination of the absorptivity (α). The emissivity (ε) was determined employing a FT-IR spectroscope fitted with an accessory for diffuse reflectance spectroscopy. Scanning Electron Microscopy (SEM) was used to characterize the microstructure and elemental chemical composition of the coatings. Coating thickness between 700nm and 20μm were obtained after deposition times shorter than 20min. Homogeneous black coatings having an absorptivity (α) higher than of 0.9 and emissivity (ε) smaller than 0.03 were obtained. Collector plates with these optical properties are suitable for solar thermal applications.